This invention relates to a coherent beam scanner for use in combination with at least one hologram.
A laser beam is usually used as a coherent beam in a coherent beam scanner, which will therefore be referred to often as a laser beam scanner hereunder. A laser beam scanner causes a laser beam to scan an output plane as a small scan spot with a hologram used as a diffraction grating for diffracting a laser beam incident thereon and with the hologram linearly moved so as to vary the angle of deflection or scan of the diffracted laser beam as will later be described with reference to one of several figures in the accompanying drawings. A sophisticated hologram for use in a laser beam scanner is defective because of unavoidable aberrations in the diffracted laser beam. It has thus been impossible to achieve a sufficiently small scan spot over a wide angle of scan. A phase plate for correcting the aberrations is described in an article contributed by Wai-Hon Lee to "Applied Optics," Volume 16, No. 5 (May, 1977), pages 1392-1399, under the title of "Holographic Grating Scanners with Aberration Corrections." The phase plate is necessary only during manufacture of the holograms. It is, however, not easy to manufacture the phase plate with a precision high enough to correct the aberrations to a satisfactory extent.